1. Field of the Invention
This invention relates to a developer charging device adapted to be used typically for an electrophotography type image forming apparatus. More particularly, it relates to a developer charging device for electrically charging the residual developer on an image carrier.
The expression of image forming apparatus as used herein refers to an apparatus adapted to form an image on a transfer material (such as paper or transparent film) as a recording medium by way of an electrophotographic image forming process. Image forming apparatus include electrophotographic copying machines, electrophotographic printers (such as LED printers and laser beam printers), electrophotographic facsimile machines and electrophotographic word processors.
2. Related Background Art
The process cartridge system is known for printers, copying machines and facsimile machines of the electrophotography type. With the process cartridge system, an electrophotographic photosensitive member (to be referred to as xe2x80x9cphotosensitive drumxe2x80x9d hereinafter), a primary charger, a developing unit and a cleaning unit are integrally combined along with other elements and put into a container to form a process cartridge, which is then removably fitted to an image forming apparatus main body.
The process cartridge system remarkably improved the operating efficiency of electrophotographic machines of the above identified type because the user of such a machine can service the process means of machine so that, as a result, the process cartridge system has become very popular. A process cartridge comprises as integral parts thereof at least one of a charging device, a developing unit or a cleaning unit and an electrophotographic photosensitive member, which is an image carrier, and can be removably fitted to an image forming apparatus main body.
A charging device is adapted to uniformly charge an electrophotographic photosensitive member with electricity to make it show a predetermined polarity and a predetermined electric potential.
Cartridges designed so as to comprise respective groups of process means that are sorted according to their service lives have been realized in order to allow each cartridge to enjoy the full service life of its own main process means. For instance, development cartridges comprising a toner containing section and a developing unit as integral parts thereof and drum cartridges comprising a photosensitive drum, a primary charger and a cleaning device as integral parts thereof are currently available.
Meanwhile, so-called cleanerless systems that do not use any dedicated cleaning device have been proposed in recent years. With an image forming apparatus, a toner image (an image formed by a developer) that corresponds to the image information given to it is formed by applying an image forming process including a step of uniformly charging the photosensitive drum with electricity, and the obtained toner image is transferred onto a transfer material such as paper or transparent film. With the cleanerless system, the toner image left on the photosensitive drum (residual toner) is removed from the latter and collected for reuse by cleaning the photosensitive drum at the time of developing the image by means of the developing unit. Then, it is possible to down-size image forming apparatus employing such a system because the cleanerless system does not involve the use of a cleaning device.
For example, Japanese Patent Application Laid-Open Publication No. 8-137368 proposes an image forming apparatus and a process cartridge that are of the cleanerless type and comprise a toner charge control means (developer charging member) arranged at a position downstream relative to the transfer charger and upstream relative to the primary charger as viewed in the rotary direction of the photosensitive drum and adapted to unify the polarities of the electric charge of the residual toner remaining on the photosensitive drum to a correct one after the transferring step so as to prevent the contact charge members of the primary charger from being smeared by the residual toner and fogged images from appearing due to a defective electric charge.
The toner charge control means as disclosed in the above identified patent document is provided with a stationary blush-shaped section that is an appropriate conductor of electricity. However, the toner charge control means can show a drawback that the residual toner is locally excessively charged if the charge of the residual toner is controlled to a proper level with the right polarity by it. Once an excessive charge appears on the residual toner, the latter no longer adheres to the contact charge members nor is collected in the developing unit because the mirror-reflection effect is too strong between the photosensitive drum and the excessively charged residual toner. Then, it is no longer possible to transfer the toner image on the transfer material by the transfer charger. As a result, the excessively charged residual toner can be fused to adhere onto the photosensitive drum and produce defective images.
It has been found that the above identified situation occurs because the stationary brush member that operates as toner charge control means is continuously held to a same position on the photosensitive drum. When the toner charge control means shows an uneven resistance, an excessive or insufficient electric charge appears at one or more than one same spots on the photosensitive drum. The problem of local excessive charge and fusion of residual toner takes place at excessively charged spots, whereas the problem of smeared contact charge members appears at insufficiently charged spots because the residual toner cannot be charged sufficiently.
Meanwhile, as a result of diversified users"" needs in recent years, a large volume of residual toner can be produced at a time particularly when photographic images that show a high printing ratio are continuously printed or when color images are printed by using a multiple developing process on the photosensitive drum so that consequently the above problems can become very remarkable.
In view of the above identified circumstances, it is therefore an object of the present invention to provide a developer charging device that can correctly and properly charge the residual developer.
Another object of the present invention is to provide a developer charging device that can prevent defective charges and defective images from being produced by the residual developer.
Still another object of the present invention is to provide a developer charging device that can effectively spread the residual developer.
Still another object of the present invention is to provide a developer charging device that can reciprocate the developer charging member in the generating line direction of the image carrier.
A further object of the present invention is to provide a developer charging device that can maintain the proper and correct positional relationship between the developer charging member and the image carrier.